Servlets Writing Filters - Java Servlets

What is Servlets Writing Filters?

Servlet Filters are Java classes that can be used in Servlet Programming for the resulting purposes −

  • To intercept needs from a client before they access a resource at back end.
  • To manipulate responses from server before they are sent back to the client.

There are various types of filters suggested by the specifications −

  • Authentication Filters.
  • Data compression Filters.
  • Encryption Filters.
  • Filters that trigger resource access events.
  • Image Conversion Filters.
  • Logging and Auditing Filters.
  • MIME-TYPE Chain Filters.
  • Tokenizing Filters .
  • XSL/T Filters That Transform XML Content.

Filters are deployed in the deployment descriptor file web.xml and then map to either servlet names or URL patterns in your application's deployment descriptor.

Once the web container starts up your web application, it makes an example of each filter that you have declared in the deployment descriptor. The filters execute in the order that they are declared in the deployment descriptor.

Servlet Filter Methods

A filter is simply a Java class that implements the javax.servlet.Filter interface. The javax.servlet.Filter interface defines three methods −

Sr.No.

Method & Description

1

public void doFilter (ServletRequest, ServletResponse, FilterChain)

This method is called by the container each time a request/response pair is passed through the chain due to a client request for a resource at the end of the chain.

2

public void init(FilterConfig filterConfig)

This method is called by the web container to indicate to a filter that it is being placed into service.

3

public void destroy()

This method is called by the web container to indicate to a filter that it is being taken out of service.

Servlet Filter − Example

Resulting is the Servlet Filter Instance that would print the clients IP address and current date time. This instance would give you basic accepting of Servlet Filter, but you can write more sophisticated filter applications using the same concept –

Compile LogFilter.java in usual way and put your class file in <Tomcat-installationdirectory>/webapps/ROOT/WEB-INF/classes

Servlet Filter Mapping in Web.xml

Filters are defined and then mapped to a URL or Servlet, in much the same way as Servlet is defined and then mapped to a URL pattern. Create the following entry for filter tag in the deployment descriptor file web.xml

The overhead filter would apply to all the servlets since we specified /* in our configuration. You can specify a specific servlet path if you want to apply filter on few servlets only.

Currently try to call any servlet in usual way and you would see made log in your web server log. You can use Log4J logger to log above log in a separate file.

Using Multiple Filters

Your web application may describe several different filters with a specific purpose. Consider, you define two filters AuthenFilter and LogFilter. Rest of the process would remain as explained above except you need to create a different mapping as stated below –

Filters Application Order

The order of filter-mapping elements in web.xml controls the order in which the web container relates the filter to the servlet. To reverse the order of the filter, you just need to reverse the filter-mapping elements in the web.xml file.

For instance, above example would apply LogFilter first and then it would apply AuthenFilter to any servlet but the following example would reverse the order –

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